Aircraft having under-wing mounted retractable landing gear assemblies are well known. In general, such landing gear configurations are comprised of a vertical leg strut connected to the axle of the landing gear wheels. The vertical leg strut is also connected to a horizontal support tube that is pivoted at its ends around its longitudinal axis in order to allow rotation of such tube and therefore movement of the landing gear between deployed and stowed positions. The ends of the support tube connect with the wing structure by means of trunnion fittings. Even though the landing gear has other points of interface with the wing structure, the trunnion fittings are the most significant interface point with the wing structure as they are the main load path from the vertical leg strut to the wing structure.
The horizontal support tube is closely aligned with the forward aircraft direction. Hence, the trunnion fittings may be referenced as front and rear trunnions in relation to the forward travel direction of the aircraft. Common aircraft design places the front trunnion relatively close to wing main box rear spar with rear trunnion further aft. Thus, a dedicated structural element to carry the rear trunnion loads is necessary according to such conventional design. The conventional landing gear design typically therefore locates a beam (or a spar) close to the rear trunnion position so that such beam will carry its loads to the main wing box structure and fuselage. Loads are reacted by the beam as shear and bending moments.
The common manufacturing process for the beam is to machine a metal billet of e.g., aluminum, steel or titanium to the designed shape. Such manufacturing fashion is referred to in this document as integral structures. The rear trunnion may be an integral part of the beam or be a fitting attached by means of fasteners. The beam may or may not be connected to upper and lower covers which form part of the external aerodynamic profile. The beam cross-sections may be, but are not limited to, “I”, “C”, “Z” or any other suitable geometries. The vertical part of the cross section is referred to as beam web. The web may or may not have vertical elements to increase its strength which are referred to as upright stiffeners. The beam web is connected at its upper and lower boundaries to horizontal elements which are referred to as upper and lower flanges.
Aircraft aerodynamic performance has been pushing designers to reduce aerodynamic profile thickness, thereby limiting available room for systems and structures. Since external loads are high and available space is limited, the structure is subjected to high internal loads. Such loads may be compressive or tensile loads depending on the part of the structure and the direction of external loads applied. These high loads occur several times throughout the aircraft service life. When high tensile loads are cyclic, they become critical for fatigue and damage tolerance. The term “damage tolerance” is hereby meant as the structure capability to withstand loads after its initial designed strength has been lowered by the presence of a crack. Such a crack is considered to occur after an initial flaw has propagated under cyclic loads present during normal aircraft operation.
The wing beam carrying the rear trunnion loads is an integral structure having low damage tolerance capabilities. Hence, in order to meet stringent maintenance requirements, designers must increase structure weight, mandate more sophisticated inspection methods, reduce inspection intervals, or any combination of such factors.
It would be therefore be desirable if wing beams carrying the under-wing landing gear trunnion loads could be provided with high damage tolerance capabilities without significantly increasing airframe structure weight. It is towards providing such a solution that the embodiments disclosed herein are directed.